This Idea For Design was originally published July 12, 1990.
Comparator hysteresis can extend the dynamic range of ac signal measurement by autoranging an rms-to-dc converter. A circuit can be built with a dynamic range of 80 dB or more, and can resolve input signals from 1 mV to 10 V with less than 1% total error (Fig. 1). The circuit also maintains high accuracy while measuring various input waveforms with different crest factors, including variable duty-cycle pulses. The autoranging method can be utilized in other signal-processing applications as well.
The AD736 rms-to-dc converter is a 1-V, full-scale device. Voltage comparator IC3 can increase the full-scale input range to 10 V automatically. When the rms-to-dc operating range is exceeded, the comparator's hysteresis switches in an attenuator at the rms-to-dc converter's input. Concurrently, it adds a gain stage to the circuit's output-buffer amplifier, maintaining a steady output voltage.
The comparator has two stable overlapping output stages. The lower hysteresis-transition point is set at 90 mV, while the upper point occurs at 1 V. This minimizes the situations in which a varying input waveform produces a confusing or unstable output state. The comparator's logic-high output is typically 4.8 V. Resistor-divider network R5 and R6 attenuate the output by a factor of 4.8, setting the noninverting input to 1 V. Similarly, the comparator's logic-low output is typically 0.44 V, setting the noninverting input to about 90 mV.
For input signals within the 100-mV to 1-V-rms operating range of IC1, the state of the switches is such that the rms-to-dc converter is driven directly from the input signal, and IC2 operates as a unity-gain follower buffering the output of IC1. When the inverting input to the comparator exceeds 1 V rms, its output goes low. The analog switches are active low, attenuator network R1 and R2, and the gain stage consisting of R3, R4, and Rg, are switched on.
R1 and R2 attenuate the input signal by a factor of 10. This means that inputs between 1 and 10 V now fall within the normal operating range of the rms-to-dc converter. The AD736's output is then amplified by 10 and the true rms value of the input signal is retained. When the output signal drops below 900 mV rms, the output of IC1 falls below 90 mV, coming within the typical range of the rms-to-dc converter again. Then, the comparator's output goes high, the input attenuator is bypassed, and the output-buffer amp functions as a voltage follower.
Due to the circuit's attenuation, the frequency response for 2 V acts similar to that of 200 mV, and 10 V acts like 1 V (Fig. 2). This helps to improve the frequency response of large input signals. The low frequency cutoff (−3 dB) is 20 Hz. With the capacitor values shown, the rms converter's settling time is 360 ms for a 1-V symmetrical sine-wave input; the settling time is greater for input signals of decreasing amplitude. Settling time improves to 36 ms when CAV = 15 µF and CF = 1 µF. The low-frequency cutoff rises 200 Hz.
All measurements shown in the figure are for sine-wave inputs, and all voltage specifications are peak values. The circuit is calibrated using a sine-wave input greater than 1 V rms. The relationship between peak and rms value is given by:
The rms value of an untrimmed sine wave = VPEAK / √2.
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